Existing switching power supplies are shown in FIG. 60 to FIG. 62. The switching power supply shown in FIG. 60 is provided with a voltage hysteresis control means. More particularly, the output of the power supply circuit is connected to the negative input of comparator 42 to amplify the error between detected voltage and reference voltage Vref. The output of comparator 42 is connected to the input of driver 47, the output of which is connected to the gate of control switch S1 and the gate of synchronous switch S2 (for example, refer to FIG. 3 in Section 7 of U.S. Pat. No. 6,147,478). A switching power supply shown in FIG. 61 is provided with a voltage mode PWM control means. More particularly, the output of said power supply circuit is connected to the negative input of error amplifier 41 to amplify the error between detected voltage and reference voltage Vref. The output of error amplifier 41 is connected to the positive input of comparator 42, the negative input of which is connected to oscillator 48, to send the triangular waveform signal from oscillator 48 to comparator 42. The output of comparator 42 is connected to the input of latch 45, the input of which is connected to oscillator 48, to send the rectangular waveform signal. Moreover, the output of latch 45 is connected to the input of driver 47, the output of which is connected to the gate of control switch S1 and the gate of synchronous switch S2 (for example, refer to FIG. 1 in Section 7 of U.S. Patent Publication No. 6147478).
A switching power supply shown in FIG. 62 is provided with a current mode PWM control means. More particularly, the negative input of error amplifier 41 is connected to the output of said switching power supply circuit to amplify the error between detected voltage and reference voltage Vref. The output of error amplifier 41 is connected to the negative input of comparator 42. The output inductor L1 is connected to current detection circuit 44, which is, in turn, connected to the positive input of comparator 42. The reset terminal of flip-flop circuit 46 is connected to comparator 42, and oscillator 48 is connected to the set terminal of flip-flop circuit 46, to send the clock signal from oscillator 48 to flip-flop circuit 46. The output of flip-flop circuit 46 is connected to the input of driver 47, the output of which is connected to the gate of control switch S1 and the gate of synchronous switch S2 (for example, refer to FIG. 2 in Sections 5 and 6 of U.S. Pat. No. 4,943,902).
A switching power supply provided with a voltage hysteresis control means uses output voltage directly to increase the inductor current by turing the switch on when output voltage drops below a specific level and reduce the inductor current by turning the switch off when output voltage becomes higher than the specific level. As output voltage is controlled by repetition of said operation, this mode provides a quick response speed. But, due to its poor operational stability, the switching power supply reacts very sensitively against the condition of the output capacitor and the load, limiting its application of usage.
Next, a switching power supply circuit provided with a voltage mode PWM control means determines the duty ratio from the fixed frequency triangular waveform signal and the amplified error signal. In this mode, operational stability is affected when the frequency difference between the fixed frequency triangular waveform signal and amplified error signal becomes close to zero. To solve the problem, the frequency band of the amplified error signal was reduced down to about 1/10 in respect to the fixed frequency triangular waveform signal.
The current mode PWM control means provides an amplified phase allowance for the amplified error signal by using the inductor current signal instead of the fixed frequency triangular waveform signal, but there remains the problem that it can not increase the frequency band of the amplified error signal significantly.
FIG. 63 shows an operational waveform diagram with a sharp increase of the load current in a switching power supply using the current mode PWM control means. FIG. 64 shows an operational waveform diagram with a sharp decrease of the load current in said switching power supply. Particularly, the upper part shows the output voltage waveform, the middle part shows the inductor current waveform, and the lower part shows the output and triangular waveform of error amplifier 41. As shown in these figures, a sharp increase of the load current reduces output voltage and, in turn, increases inductor current, while a sharp decrease of the load current increases the output voltage considerably and, in turn, decreases the inductor current. However, as more than few cycles are required to stabilize the output voltage, there was the problem that the response speed of the system delays to obtain stable power supply operation.
The present invention, which is made considering the aforesaid problems, provides a switching power supply which ensures the stability with no need of lowering the frequency band of the amplified error signal.
Also, the invention provides a new switching power supply which materializes stable output ripple characteristics.
Furthermore, the invention provides a new switching power supply which materializes stable oscillation frequency and output ripple characteristics.